There are many types of shared wireless communications networks, such as those used in Third Generation (3G) cellular telephony communications. One of the techniques used in wireless communications equipment using a shared network includes the allocation of transmit power for communications in different channels. By effectively controlling the transmit power, it is possible to reduce overall power consumption, increase utilization of bandwidth and maintain at the appropriate value the signal-to-interference ratios (SIRs) of the different physical channels supporting a coded composite transport channel (CCTrCH).
In certain current wireless systems, the different types of data a user needs to transmit or receive may be coded and multiplexed in one or more CCTrCHs. The multiplexing is performed in a way that the quality of service (QoS) for those different types of data, in terms of the probability of receiving a transport block in error, will be met for the same value of the SIR of the received symbols of the CCTrCH. This allows an optimal use of the radio resources. These systems are able to transmit a wide range of services, from high data rate services such as video and Internet downloads, to low data rate services such as voice.
Referring to FIG. 1, a plurality and variety of user services are graphically shown as individual data streams. These individual data streams are assigned to transport channels A, B and C, whereby the data streams are coded and multiplexed. Each transport channel A, B, C is assigned a specific coding rate and a specific transmission time interval (TTI). The coding rate for each transport channel determines the number of transmitted bits of the physical layer, and the TTI defines the delivery period of the block of data to be transmitted. For example, the TTI may be 10, 20, 40 or 80 ms. Coded bits from the various transport channels are multiplexed and interleaved to form a CCTrCH. The bits of a CCTrCH are then paired to form symbols, which are transmitted (after spreading) through one or a plurality of physical channels defined in terms of time slot and spreading code.
The transmission on physical channels occurs after the transport channels have been multiplexed onto the CCTrCH. The number of symbols (Ns) carried by a physical channel is inversely proportional to the spreading factor of the code of the physical channel. In other words, Ns=Nc/G, where Nc is the number of chips spreading the symbols in a time slot, and G is the spreading factor. The number of chips Nc is normally the same for all physical channels supporting a CCTrCH.
Referring to FIG. 2, at the transmitter side each physical channel is transmitted at a certain power level Pi, where the index i is over the physical channels. At the receiver side, the signal from a physical channel has a power level Ri=Pi/L where L is the path loss. After despreading, the power of the received symbols is Gi*Pi/L, where Gi is the spreading factor of the physical channel. Therefore, if the interference level is Ii in the slot occupied by the physical channel, the SIR in this physical channel, at the symbol level, is given by Equation (1):
                              SIR          i                =                                            G              i                        ⁢                          P              i                                            LI            i                                              Equation        ⁢                                  ⁢                  (          1          )                    
The quality of reception of a CCTrCH, in terms of the probability of receiving a transport block in error, is a function of the SIRs of the received symbols. When the SIRs of the received symbols all have the same value, this value is a direct indicator of the quality of the connection. In general, however, the SIRs of received symbols may have different values. An approximate indicator of the quality of the connection may be obtained by averaging the values of the SIRs of the received symbols. In essence, the quality of the connection with unequal SIRs of symbols, whose average value is SIR, should be approximately the same as the quality of a connection with the SIRs of symbols all equal to SIR.
The averaging can be done linearly or logarithmically (i.e., averaging the SIR values in dB). The logarithmic average is always lower than the linear average, and as such, can be considered a more conservative indicator of the quality of the connection. The computations can be used to provide linear or logarithmic averages, as a function of the different power levels of the physical channels of the CCTrCH.
To perform a linear average, the average SIR of the CCTrCH ( SIRlin) is computed as follows:
                                          SIR            _                    lin                =                                            ∑                              i                =                1                            N                        ⁢                                                  ⁢                                          (                                  Number                  ⁢                                                                          ⁢                  of                  ⁢                                                                          ⁢                  symbols                  ⁢                                                                          ⁢                  in                  ⁢                                                                          ⁢                  physical                  ⁢                                                                          ⁢                  channel                  ⁢                                                                          ⁢                  i                                )                            ×                              SIR                i                                                          Total            ⁢                                                  ⁢            number            ⁢                                                  ⁢            of            ⁢                                                  ⁢            symbols            ⁢                                                  ⁢            in            ⁢                                                  ⁢            all            ⁢                                                  ⁢            physical            ⁢                                                  ⁢            channels                                              Equation        ⁢                                  ⁢                  (          2          )                    The SIR of each physical channel is multiplexed by the number of symbols it is carrying to compute the average over all symbols of the CCTrCH. Since the number of symbols in physical channel i is equal to Nc/Gi, and substituting Equation (1) for SIRi, this becomes:
                                                                                          SIR                  _                                lin                            =                                                                    ∑                                          i                      =                      1                                        N                                    ⁢                                                            (                                                                        N                          c                                                /                                                  G                          i                                                                    )                                        ×                                          (                                                                        G                          i                                                ⁢                                                                              P                            i                                                    /                          L                                                ⁢                                                                                                  ⁢                                                  I                          i                                                                    )                                                                                                            ∑                                          i                      =                      1                                        N                                    ⁢                                                                          ⁢                                      (                                                                  N                        c                                            /                                              G                        i                                                              )                                                                                                                          =                                                                    ∑                                          i                      =                      1                                        N                                    ⁢                                                                          ⁢                                                            P                      i                                                              I                      i                                                                                        L                  ⁢                                                            ∑                                              i                        =                        1                                            N                                        ⁢                                                                                  ⁢                                          1                                              G                        i                                                                                                                                                    Equation        ⁢                                  ⁢                  (          3          )                    
Equation (3) expresses the linear average SIR ( SIRlin)of the CCTrCH as a function of the transmission power levels (Pi), interference levels (Ii) and spreading factors (Gi) of all physical channels, as well as the path loss (L).
The logarithmic average SIR ( SIRlog) of the CCTrCH is defined, following similar principles, as follows:
                                                                                          log                  10                                ⁡                                  (                                                            SIR                      _                                        log                                    )                                            ⁢                            =                                                                    ∑                                          i                      =                      1                                        N                                    ⁢                                                                          ⁢                                                            (                                              Number                        ⁢                                                                                                  ⁢                        of                        ⁢                                                                                                  ⁢                        symbols                        ⁢                                                                                                  ⁢                        in                        ⁢                                                                                                  ⁢                        physical                        ⁢                                                                                                  ⁢                        channel                        ⁢                                                                                                  ⁢                        i                                            )                                        ×                                                                  log                        10                                            ⁡                                              (                                                  SIR                          i                                                )                                                                                                              Total                  ⁢                                                                          ⁢                  number                  ⁢                                                                          ⁢                  of                  ⁢                                                                          ⁢                  symbols                  ⁢                                                                          ⁢                  in                  ⁢                                                                          ⁢                  all                  ⁢                                                                          ⁢                  physical                  ⁢                                                                          ⁢                  channels                                                                                                                      ⁢                              =                                                                            ∑                                              i                        =                        1                                            N                                        ⁢                                                                  (                                                                              N                            c                                                    /                                                      G                            i                                                                          )                                            ×                                                                        log                          10                                                ⁡                                                  (                                                                                    G                              i                                                        ⁢                                                                                          P                                i                                                            /                              L                                                        ⁢                                                                                                                  ⁢                                                          I                              i                                                                                )                                                                                                                                                ∑                                              i                        =                        1                                            N                                        ⁢                                                                                  ⁢                                          (                                                                        N                          c                                                /                                                  G                          i                                                                    )                                                                                                                                                            ⁢                              =                                                                            ∑                                              i                        =                        1                                            N                                        ⁢                                                                  (                                                  1                          /                                                      G                            i                                                                          )                                            ×                                                                        log                          10                                                ⁡                                                  (                                                                                    G                              i                                                        ⁢                                                                                          P                                i                                                            /                              L                                                        ⁢                                                                                                                  ⁢                                                          I                              i                                                                                )                                                                                                                                                ∑                                              i                        =                        1                                            N                                        ⁢                                                                                  ⁢                                          (                                              1                        /                                                  G                          i                                                                    )                                                                                                                              Equation        ⁢                                  ⁢                  (          4          )                    
Equation (4) expresses the logarithmic average SIR of the CCTrCH as a function of the transmission power levels (Pi), interference levels (Ii) and spreading factors (Gi) of all physical channels, as well as the path loss (L).
In most current wireless systems the downlink power control is closed-loop. This means that the base station must adjust the transmit power every frame based on up/down transmit power control (TPC) commands sent by the mobile unit during an uplink transmission, (for example, uplink CCTrCH). The mobile unit determines the TPC command by comparing the experienced SIR to a certain SIR target. While the downlink CCTrCH of a mobile unit may have physical channels occupying more than one slot, multiple TPC commands per frame would be possible only in case of multiple uplink CCTrCHs. In many cases however, there is only a single uplink CCTrCH for a mobile unit. In this situation, there is only one TPC command per frame that the mobile unit can send to command the power transmitted by the base station on more than one slot.
Since the interference signal code power (ISCP) on each downlink slot is subject to variations over time, the downlink CCTrCH may be adversely affected. For example, assuming physical channels on 2 slots, over a certain period of time the ISCP on the first slot may increase by 5 dB while the ISCP on the second slot may decrease by 3 dB. Using a single TPC command to control the transmission power on those two slots, it is impossible for the mobile unit to inform the base station to increase the power on one slot but decrease the power on the other slot. As a result, the SIRs of the downlink physical channels occupying different slots are very likely to drift apart if the base station strictly follows the TPC commands sent by the mobile unit, because the base station has to apply the same TPC command to all physical channels regardless of the slots they are occupying.
It is desirable for an optimal use of the radio resources that the SIRs of the different physical channels be as equal as possible at the symbol level. To achieve this, and because interference conditions in the different time slots change over time, the system needs to readjust, from time to time, the transmission powers allocated in each timeslot to the different physical channels so that the SIRs of the physical channels which are in different slots become as equal as possible. This process is known as SIR equalization and is achieved through the process shown in FIG. 3.
FIG. 3 shows the process implemented by a Controlling Radio Network Controller (CRNC), a base station and a mobile unit to perform SIR equalization. This process enables the base station to use the downlink timeslot ISCP values when deciding the downlink TX power for each timeslot. The mobile unit periodically measures downlink (DL) ISCP and transmits ISCP measurements to the CRNC for each timeslot in which it is receiving a signal. The CRNC sends a DL POWER TIMESLOT CONTROL REQUEST message to the base station, along with DL ISCP values, which are the interference levels in every slot occupied by the physical channels for the concerned CCTrCH. Upon reception, the base station uses the indicated DL timeslot ISCP values sent by the CRNC to set the downlink TX power for each timeslot. The base station reduces the downlink TX power in those downlink timeslots of the radio link where the interference is low; and increases the downlink TX power in those timeslots where the interference is high, while keeping the total downlink power in the radio link unchanged.
The procedure 30 followed by the base station for performing SIR equalization is detailed in FIG. 4. The procedure 30 starts with the reception of the DOWNLINK POWER TIMESLOT CONTROL REQUEST message from the CRNC containing the ISCP values (step 32). The base station associates interference levels Ii to the different physical channels, where the index i is over the physical channels, depending on the time slot occupied by each of the physical channels (step 34). The interference level I is the same for all physical channels that occupy the same time slot, (i.e., Ii=Ij if physical channels i and j are in the same time slot.) Since the base station is responsible for the transmission of the signal to the mobile unit, it always knows the latest transmitted power level Pi as well as the spreading factor Gi of every physical channel.
The base station then takes these set of values (I1, I2, . . . , IN, P1, P2, . . . , PN, G1, G2, . . . , GN), where N is the number of physical channels supporting the CCTrCH, and computes a new set of values (Pi′, P2′, . . . , PN′) for the transmission power levels of the physical channels (step 36). The goal of equalization is to make the SIR of all physical channels equal. Accordingly;
                              SIR          i          ′                =                                                            G                i                            ⁢                              P                i                ′                                                    LI              i                                =          K                                    Equation        ⁢                                  ⁢                  (          5          )                    In Equation (5), SIR′i denotes the SIR of physical channel i just after equalization, and K is the value of the SIR after equalization, which must be the same for all physical channels. In one prior art system, this value K is computed according to the following:
                    K        =                                            ∑                              i                =                1                            N                        ⁢                                                  ⁢                          P              i                                            L            ⁢                                          ∑                                  i                  =                  1                                N                            ⁢                                                          ⁢                                                I                  i                                                  G                  i                                                                                        Equation        ⁢                                  ⁢                  (          6          )                    Substituting Equation (6) in Equation (5), the new set of transmission power values (P1′, P2′, . . . , PN′) is therefore computed by applying the following equation:
                              P          i          ′                =                              (                                                            ∑                                      i                    =                    1                                    N                                ⁢                                                                  ⁢                                  P                  i                                                                              ∑                                      i                    =                    1                                    N                                ⁢                                                                  ⁢                                                      I                    i                                                        G                    i                                                                        )                    ⁢                                    I              i                                      G              i                                                          Equation        ⁢                                  ⁢                  (          7          )                    Immediately after application of Equation (7), the SIRs of the physical channels are all equal. In addition, it can be verified that the sum of the transmission powers is the same before and after SIR equalization
      (                            ∑                      i            =            1                    N                ⁢                  P          i          ′                    =                        ∑                      i            =            1                    N                ⁢                                  ⁢                  P          i                      )    .These new power values (Pi′, P2 ′, . . . PN′) are then applied to the physical channels (step 38).
While the process shown in FIG. 4 and set forth in Equation 7 equalizes the SIRs of the different physical channels, it suffers from a major drawback. Although, the total power over all physical channels before and after SIR equalization is the same, the SIR of the physical channels after application of the new set of transmission powers Pi′ could be significantly different from the average SIR over all physical channels before equalization, as defined according to either Equation (3) (linear average) or Equation (4) (logarithmic average). As a result, the quality of the reception may suffer a sudden and severe degradation until power control eventually restores the average SIR to its original level.
Therefore, there are instances in which the existing equalization process does not maintain the average SIR constant. By the way of example, the CCTrCH may be supported by two physical channels of equal spreading factors, (such as G1=G2=16). The transmission power levels of the physical channels before equalization are P1=P2=1 mW. The corresponding interference levels are: I1=1×10−9 mW and I2=8×10−9 mW. The path loss is L=1×109. Prior to SIR equalization, the SIRs of physical channels are therefore: SIR1=16 and SIR2=2. The average SIR (linear) is SIRlin=9 according to Equation (3). The average SIR (logarithmic) is SIRlog=5.7. After SIR equalization, Equation (7) shows that the new transmission power levels are: P1′=0.22 mW and P2′=1.78 mW, and the SIRs of both physical channels (as well as the average SIR, linear or logarithmic) is equal to SIRhd lin= SIRlog=SIR1=SIR2=3.56. Clearly, this is lower than both the linear or logarithmic average SIR before the SIR equalization procedure. If the average SIR, either linear or logarithmic, was at a level such that the QoS was just met for this CCTrCH, this reduction would result in a degradation of quality until power control restores the average SIR to its original level. This behavior is undesirable.